Magnetic and optical disks require precision surfaces with extremely low defect rates to function properly. A typical magnetic disk comprises a substrate on which multiple layers of various materials are deposited. For example, a glass or aluminum substrate might be coated with thin films of Cr as an undercoat, a cobalt alloy magnetic layer, and a hydrogenated carbon overcoat. Depending on the stage of the process, these surfaces are not necessarily uniform. For example, a small circular band on the surface of the disk may be textured using a laser to form microscopic bumps. This textured region is intended to provide a low stiction area for the sliders to rest during nonoperating periods. In addition to intentional variations, there may be various types of defects. As the disks progress through the manufacturing process, various tests and inspections are used to detect defective disks so that they may either be reworked or discarded. In addition to visual inspections, a disk may be subjected to glide tests which are sensitive to the flatness of the planar surfaces, as well as magnetic read/write tests. Due to high capacities of magnetic disks, it is typically not practical to magnetically test each bit which can be stored on the disk. Laser surface inspection of the disks, if sufficiently precise, may actually be superior to current magnetic tests in detecting defects. Magnetic defects are usually associated with visible defects, but the visible defects can be detected more efficiently through laser inspection even though the laser spot size is considerably larger than the area in which a bit can be recorded. Thus, laser inspection allows greater test coverage of the disk in a cost-effective manner.
It is also desirable to inspect the disk substrate before any coatings or additional processing steps are added. If surface defects can be detected at this stage, a great savings can be realized by not incurring the costs of additional processing on defective parts.
There are several problems associated with laser-based testing of glass or other light-transmissive disk substrates. First, the glass substrate has a very low reflectivity (approximately 4%), which results in only a small signal amplitude. Secondly, the second surface of the glass substrate also has a very low reflectivity of only approximately 4%, which means that since the transmission through the glass disk is nearly 100%, the second surface produces a reflected beam that interferes with the reflection from the first surface. This results in an unwanted interference fringe pattern in the output.
What is needed is a reliable laser-based surface inspection tool for light-transmissive disk substrates.